1. Field of the Invention
The invention relates to a lithium-containing, transparent glass-ceramic material with low thermal expansion, having a very largely amorphous, lithium-depleted, predominantly vitreous or glassy surface zone; it relates to a method by which transparent, glass-ceramic materials of this type which are smooth on all sides and exhibit surprisingly high transmission and strength can be produced, and to the use of this glass-ceramic material of the invention.
2. Description of Related Art
Transparent glass-ceramic materials with a weakly pronounced intrinsic color, or almost none at all, have nowadays become increasingly important. They should contribute the properties either of coatability/decorability, or those of very high transmission, resulting from a low intrinsic color and a very slightly pronounced scattering. At the same time, high strength is required for the various applications.
The prior art has disclosed a multiplicity of methods for producing solid glass-ceramic materials having low thermal expansion. Such glass-ceramics are employed especially in those areas which are characterized by severe temperature fluctuations and/or mechanical loading, such as, for example, in the form of cookware, as a glass-ceramic cooktop, or in the area of fire protection.
Glass-ceramic materials are produced generally in three steps from a precursor glass or green glass. The “green” glass, shaped to form an article, is heated to a nucleation temperature, where a multiplicity of crystal nuclei are generated in the “green” glass with the aid of nucleating agents. Thereafter the temperature is raised to an optimum crystallization temperature, at which crystal growth takes place on the crystal nuclei formed. The article thus produced is subsequently cooled to room temperature in a variety of ways.
Enhanced strength in a glass-ceramic material is generally achieved by generating different states of stress in the surface and in the interior of the glass-ceramic. For this purpose, the surface of the glass-ceramic material is placed under compressive stress. If a mechanical load then occurs, the surface under compressive stress then prevents the shattering of the glass-ceramic material, up to a limit which is dependent on the material and/or on the production method.
The above-described compressive stress in the surface can be generated by a wide variety of methods. Presented hereinafter are some of the prior-art methods similar to the present invention.
U.S. Pat. No. 3,773,856 discloses a method for producing transparent glass-ceramics with high strength and good thermal shock resistance, using a nucleating agent which changes its valence under a reducing atmosphere, the nucleating agent with the changed valence exhibiting an increased nucleation efficacy. Accordingly, the nucleating step in ceramicization takes place under a reducing atmosphere which is in direct contact with the surface of the starting glass, so that the nucleating agent is reduced at and directly beneath the surface of the glass and hence exhibits an increased efficacy. Then, in the course of the subsequent crystallizing step, the degree of crystallization at the surface of the article is greater than in the interior. The difference in degree of crystallization between, surface and interior of the article results in the desired compressive stress in the article's surface. The reduction, of the nucleating agent can be catalysed by the addition of small amounts of certain metal oxides, such as copper oxide, iron oxide, or manganese oxide, for example. Nucleating agents of the invention are TiO2, ZrO2, or mixtures thereof.
U.S. Pat. No. 4,391,314 discloses a method which, for the purpose of generating the compressive stress in the surface of a glass-ceramic, makes use of differences in thermal expansion coefficient between crystal phase (lithium aluminum silicate crystal phase) and residual glass phase (borosilicate or boroaluminosilicate). Above 500 to 750° C., the residual glass phase dominates the thermal expansion coefficient of the glass-ceramic, since in this phase the crystals formed “float” freely in the matrix of the residual glass phase. As cooling progresses, the contraction of the residual glass phase is substantially greater than that of the crystal phase, and ultimately leads to the crystals coming into contact with one another, while the residual glass phase is only still present interstitially, in pockets. As a result, of progression in cooling, there are compressive stresses or “sticking” at the grain boundaries of the crystals.
The method which is described in U.S. Pat. No. 3,524,748 is based on the differences in thermal expansion coefficient between alpha-quartz and beta-quartz. Beta-quartz has a lower thermal expansion coefficient than alpha-quartz, and forms the principal crystal phase of the surface layer, which is under compressive stress, while beta-quartz constitutes the principal crystal phase, of the interior of the glass-ceramic article. In order to produce such glass-ceramic articles, a starting glass is melted, shaped, subjected optionally to nucleation, and heated to temperatures between 1000 and 1250° C. The residence time at these temperatures is long enough to ensure a uniform crystallization whose principal phase is beta-quartz. Quoted for example for the nucleation at 800 to 950° C. are one to eight hours; for the crystallization phase, two to eight hours. The surface of the article is subsequently quenched within from 10 to 60 seconds to temperatures below 573° C., in order to prevent the beta-quartz produced converting back into alpha-quartz near the surface. The interior of the glass-ceramic article is cooled more slowly, and so there is a conversion into alpha-quartz there. Owing to the higher thermal expansion coefficient of alpha-quartz, the interior of the glass-ceramic article contracts to a greater extent on cooling than does the beta-quartz surface layer, and hence generates a compressive stress in the surface layer. Glass compositions suitable for the production of glass-ceramic articles of this kind are those comprising SiO2, Al2O3, MgO2, and ZrO2 as principal components. There may be up to 15% by weight of ZnO present, and also small amounts of Li2O, Nb2O3, Ta2O2, CaO, BaO, TiO2, and B3O3, with the latter compounds accounting for not more than 4% by weight in total.
U.S. Pat. No. 4,940,674 teaches the production of transparent, substantially hazing-free glass-ceramic articles with a low thermal expansion coefficient, the principal components of their starting glasses including Li2O, Al2O3, SiO2, and B2O3, with the possible addition thereto of TiO2 and/or ZrO2 as nucleating agents, and of very small amounts of Cr2O3 as a specialty nucleating agent for the hazing-free production of transparent glass-ceramics. In addition, there are small amounts of glass-modifying oxides of the group K2O, SrO and/or BaO.
Patent specification DE 101 10 225 C2 discloses a glass-ceramic, support material for coating with a mirror layer, comprising glass-ceramic and a vitreous surface layer which comprises at least Na2O and/or K2O and has a surface roughness, without polishing, of Ra<50 nm. The composition there is achieved by the formation of a vitreous surface layer which is enriched with the components Na2O/K2O and has a thickness of up to 1.5 μm. The effect, of the vitreous surface layer is that the increase in surface roughness in the glass-ceramic relative to the starting glass is less than 10 nm.